The central offices of Public Switched Telephone Networks employ network hybrids to interface analog subscribers to the local exchange. Typically, a 2-wire full-duplex line connects the subscriber to the central office while the network hybrid provides an interface to the two 2-wire simplex digital transmission network. In this way, data are transmitted to and received from other subscribers who are connected via a 2-wire line to the digital transmission network by another hybrid at the local exchange.
As is known in the art, echo cancellation is often discussed with reference to speech signal communication between a “near end” and a “far end” and vice versa. A person speaking at the “far end” of a telephone connection has her speech sent over the digital transmission network to a person listening (and eventually speaking) at the “near end.” A portion of the speech signal that is transmitted from the far end speaker “echoes” off of the network hybrid that is farther away from the far end speaker and closer to the person located at the near end (the near end speaker). In this way, the “near end” hybrid creates an unwanted signal echo of the transmitted far end speech signal and sends it back toward the far end, only to be heard by the far end speaker as an annoying echo of her own voice.
As is known in the art, network echo canceller circuitry is located at both network hybrids (one at the “far end,” and one at the “near end”). In the case of far end speaker echo, that is, the echo of a transmitted far end speech signal back to the far end, a network echo canceller at the far end network hybrid (the hybrid closest to the far end speaker) is used to cancel the echo. The echo canceller at the far end generates a local replica of the echo generated by the far end signal as it passes through an echo path, or echo channel at the near end. This local replica of the echo is generated and used by the echo canceller to attempt to cancel the echo before it returns to the far end speaker. The echo path or channel is the entire path traveled by the transmitted far end signal as it leaves the far end hybrid, as a portion of it echoes off of the near end hybrid, and as its echo returns back to the far end hybrid. In particular, the echo path represents the outgoing and incoming digital transmission lines as well as the near end network hybrid (the one closest to the near end speaker, and distant from the far end speaker).
Similarly, in the case of echo from the near end speaker, that is, the echo of a transmitted near end speech signal back to the near end, a network echo canceller at the near end network hybrid (the hybrid closest to the near end speaker) cancels the echo that is generated from a portion of the near end speech signal echoing off of the opposite hybrid (the far end hybrid) and that returns toward the network hybrid closest to the near end (the near end hybrid).
A typical network echo canceller employs an adaptive digital transversal filter to model the impulse response of the unknown echo channel so that the echo signal can be cancelled. The echo impulse response coefficients used in the transversal filter are updated to track the characteristics of the unknown echo channel. For reference purposes, network echo cancellers are examined from the point of view of the far end speaker, that is, these cancellers act to minimize far end echo.
Generally speaking, the presence of near end speech interferes with the ability of the adaptive transversal filter of the network echo canceller to properly and accurately model the characteristic response of the unknown echo channel via the coefficient updating process. Near end speech is present when only the near end speaker is talking or when the near end speaker and the far end speaker are conversing simultaneously. This latter situation is referred to as “doubletalk.” If speech is present from the near end speaker, this speech will be combined with any far end speaker echo that is present and will be sent toward the echo canceller at the far end telephone company central office. This near end speech poses a problem for coefficient updating and adaptation of the digital transversal filter.
If coefficient updating is allowed to continue while near end speech is present, the updated echo impulse response coefficients will no longer reflect the characteristic response of the echo channel. The filter coefficients will instead be corrupted by the samples of the near end speech signal samples that are now combined with the samples of the far end echo signal.
Prior attempts at avoiding or limiting the corruption of the echo impulse response or filter tap coefficients due to the presence of near end speech have involved techniques such as freezing or disabling digital filter adaptation when near end speech is detected, indicated, or declared. These techniques imply that the network echo canceller includes a near end speech detector or indicator. There is a delay time associated with near end speech indication that is significant since the adaptive transversal filter is not immediately disabled and/or the filter tap coefficients are not frozen in time to avoid corruption.
Needed is a computationally efficient and easily implemented technique to preserve and protect the echo impulse coefficients used in adaptive transversal filtering from corruption and distortion due to the presence of near end speech and to compensate for the delay time associated with the detection or indication of the near end speech.